Symptoms of abnormal heart rhythms are generally referred to as cardiac arrhythmias, with an abnormally rapid rhythm being referred to as a tachycardia. The present invention is concerned with the treatment of tachycardias which are frequently caused by the presence of an "arrhythmogenic site" or "accessory atrioventricular pathway" close to the inner surface of the chambers of a heart. The heart includes a number of normal pathways which are responsible for the propagation of electrical signals from the upper to lower chambers necessary for performing normal systole and diastole function. The presence of arrhythmogenic site or accessory pathway can bypass or short circuit the normal pathway, potentially resulting in very rapid heart contractions, referred to here as tachycardias.
Treatment of tachycardias may be accomplished by a variety of approaches, including drugs, surgery, implantable pacemakers/defibrillators, and catheter ablation. While drugs may be the treatment of choice for many patients, they only mask the symptoms and do not cure the underlying causes. Implantable devices only correct the arrhythmia after it occurs. Surgical and catheter-based treatments, in contrast, will actually cure the problem, usually by ablating the abnormal arrhythmogenic tissue or accessory pathway responsible for the tachycardia. It is important for a physician to accurately steer the catheter to the exact site for ablation. Once at the site, it is important for a physician to control the emission of energy to ablate the tissue within the heart.
Of particular interest to the present invention are radiofrequency (RF) ablation protocols which have been proven to be highly effective in tachycardia treatment while exposing a patient to minimal side effects and risks. RF catheter ablation is generally performed after conducting an initial mapping study where the locations of the arrhythmogenic site and/or accessory pathway are determined. After a mapping study, an ablation catheter is usually introduced to the target heart chamber and is manipulated so that the ablation tip electrode lies exactly at the target tissue site. RF energy or other suitable energy is then applied through the tip electrode to the cardiac tissue in order to ablate the tissue of arrhythmogenic site or the accessory pathway. By successfully destroying that tissue, the abnormal signal patterns responsible for the tachycardia may be eliminated.
The impedance usually rises at the tissue contact site when RF energy is delivered through an electrode. To create a deeper and larger lesion, the surface of the tissue contact sites need to maintain a proper temperature by a cooled fluid irrigation or infusion to partially compensate for the temperature rise due to RF energy delivery. The following U.S. patents have disclosed use of plurality of irrigation ports in different manners to cool the tissue contact surface. In practice, the fluid coming out of the irrigation ports may not evenly cover all the surface area of the electrode or the tissue to be ablated. Those patents are U.S. Pat. No. 5,545,161 to Imran, U.S. Pat. No. 5,462,521 to Brucker et al., U.S. Pat. No. 5,437,662 to Nardella, U.S. Pat. No. 5,423,811 to Imran et al., U.S. Pat. No. 5,348,554 to Imran et al., and U.S. Pat. No. 5,334,193 to Nardella. However, none of the above discloses an irrigation system of cooled fluid through a porous means to form a uniform protective fluid layer around the electrode.
The tip section of a catheter is referred to here as the portion of that catheter shaft containing at least one electrode. In one embodiment, a catheter utilized in the endocardial RF ablation is inserted into a major vein or artery, usually in the neck or groin area. The catheter is then guided into an appropriate chamber of the heart by appropriate manipulation through the vein or artery. The tip of a catheter must be manipulatable by a physician from the proximal end of the catheter, so that the electrodes at the tip section can be positioned against the tissue site to be ablated. The catheter must have a great deal of flexibility in order to follow the pathway of major blood vessels into the heart. It must permit user manipulation of the tip even when the catheter body is in a curved and/or twisted configuration. The tip section of a conventional electrophysiology catheter that is deflectable usually contains one large electrode about 4 mm in length for ablation purpose. The lesion is generally not deep because of potential impedance rise of tissue in contact with the catheter electrode and the ablation time needs to be cut short. Even in the case of a conventional catheter with irrigation capabilities by utilizing a plurality of irrigation ports, the cooled fluid do not evenly and uniformly rinse the ablation electrodes.
After the exact location of a target tissue is identified, the ablation catheter may still not easily approach the target site even with assistance of an internal viewing means. This viewing situation may turn into a nightmare when an internal viewing approach becomes prohibitive or unavailable during procedures. An external ultrasonic imaging capability therefore becomes in need so that ablation is not taking place in an inappropriate location. The fluoroscope time can be substantially cut short when an external ultrasonic imaging is used instead. In the U.S. Pat. No. 4,794,931, there has been disclosed a catheter and system which can be utilized for ultrasonic imaging. However, there is no disclosure to how such a catheter and system can be utilized in conjunction with an endocardial or epicardial ablation catheter having a porous shaft with irrigation capabilities to achieve the desired ultrasonic imaging and ultimately the desired ablation.
While a radiofrequency electrophysiology ablation procedure using an existing catheter has had promising results, the tip section of a known catheter usually have only a plurality of fluid infusion ports which may not evenly rinse the electrode when contacting the tissue for ablation purpose. Therefore there is a need for a new and improved catheter for making a deeper and larger lesion in the cardiac tissue.